WORCESTER, Mass. – When doctors told Wanda Ford her 2-year-old son had lead poisoning, she never suspected that the backyard in her low-income neighborhood was the likely culprit.

Ford knew that exposure to the heavy metal could be dangerous. So when she and her husband moved into the Lower Lincoln Street neighborhood, Ford, then pregnant, took steps to make sure their 100-year-old home was lead-free.

“We never thought to test the soil – my son played in the backyard all the time,” said Ford, whose son is now seven.

It’s long been known that children in poorer neighborhoods like Ford’s are more likely to be exposed to lead, industrial emissions, vehicle exhaust and other contaminants. Now, scientists are beginning to suspect that low-income children aren’t just more exposed – they actually may be more biologically susceptible to them, even at low levels.

A growing body of research suggests that the chronic stressors of poverty may fundamentally alter the way the body reacts to pollutants, especially in young children. Several studies have found that such stress may exacerbate the effects of lead on children’s developing brains, while others reported more asthma symptoms in kids with simultaneous exposure to air pollution and socioeconomic problems.

Everyone experiences stress occasionally; it can improve focus and performance to overcome obstacles at work, during athletic competitions, or in everyday life. But stress also can harm the body.

“When the stress is chronic and the stressors are out of our control, we experience it as a threat rather than a challenge,” said Dr. Rosalind Wright, a physician and epidemiologist at the Harvard School of Public Health. “This type of stress can have negative, lasting effects on key systems in the body. It’s like having the fight or flight response turned on all the time.”

Schools and homes next to refineries and Superfund sites, farm workers drinking toxicwater, urban children breathing exhaust from congested streets. Many of these people are living in poverty or with low incomes, and they have to cope with socioeconomic stress as well as high exposure to pollutants.

Scientists say the chronic stress of living in such areas and facing financial strain, racial issues and high crime rates can wear down the systems responsible for controlling immunity and hormones. Hormone levels needed for proper brain development may be altered, or the immune system may continually release inflammatory molecules into the blood.

“This may make you more susceptible to everything else around you, including pollution,” said Jane Clougherty, an exposure scientist and epidemiologist at the University of Pittsburgh Graduate School of Public Health.

One + one = four

Stress, when combined with certain pollutants, may produce a much greater health effect than either stress or pollution alone.

“It would be like adding one and one together and getting three or four,” said Dr. Ted Schettler, science director of the Science and Environmental Health Network, a non-profit organization focused on applying science to promote health.

“Socioeconomic status may affect underlying biology, making exposure to certain chemicals more adverse for the poorer kid,” he said.

In Worcester, about 40 miles west of Boston, nearly one in five residents lives below poverty level, almost double the Massachusetts average, according to census data from 2010. Its median household income is roughly 30 percent lower than the state’s.